It is well known that for cemented carbide cutting tool inserts used in the machining of cast irons, the cutting edge is worn by different wear mechanisms such as chemical and abrasive wear but the cutting edge is generally also subjected to crack formation due to the intermittent cutting load, resulting in so called chippings and edge fractures caused by different types of cracks in the inserts.
Different types of crack patterns may appear during machining of cast irons. One important type is the so called comb cracks, which are formed perpendicularly to the cutting edge. The formation of comb cracks is strongly influenced by the cooling conditions during cutting. In particular, the use of fluid coolant increases the tendency to form comb cracks, often also called thermal cracks. The use of fluid coolant leads to large temperature gradients and thermal tensile stresses in the insert surface, increasing the tendency for formation of surface cracks, in particular in the case of coated cutting tool inserts where the hard but brittle ceramic surface coating is prone to crack under conditions involving unfavourable thermal tensile stresses. Cracks in the coating increases the risk for chipping and edge fractures and for flaking of the coating.
Characteristic for cast irons is the so called surface skin, the surface zone of the cast component often contains a structure which deviates considerably from the bulk structure and also contains hard inclusion and sand from the mould. In this case, a coated cemented carbide insert must be used including a substrate with the proper toughness of the cemented carbide grade and on the surface a wear resistant refractory coating.
Furthermore, different cutting conditions such as cutting speed, depth of cut, cutting feed rate and also external factors such as vibrations of the work piece and the above mentioned surface zone in iron casting, etc., require a plurality of different properties of the cutting edge.
Commercial cemented carbide tool inserts for milling of cast irons under wet conditions are usually optimised with respect to one or two of the wear types observed.
U.S. Pat. No. 5,912,051 discloses a coated cutting insert particularly useful for dry milling of grey cast iron.
U.S. Pat. No. 5,863,640 discloses a coated turning insert particularly useful for intermittent turning in low alloyed steel.
In U.S. Pat. No. 6,062,776 is disclosed a coated cemented carbide cutting tool particularly designed for the wet and dry milling of workpieces of low and medium alloyed steels or stainless steels, with or without abrasive surface zones, in machining operations requiring a high degree of toughness of the carbide cutting edge. The external cutting conditions are characterised by complex shapes of the workpiece, vibrations, chip hammering, recutting of the chips etc.
In U.S. Pat. No. 6,177,178 is disclosed a coated cemented carbide cutting tool particularly designed for the wet and dry milling of low and medium alloyed steels.
WO 01/16388 discloses a coated insert particularly useful for milling in low and medium alloyed steels with or without abrasive surface zones during dry or wet conditions at high cutting speed, and milling hardened steels at high cutting speed.